Line type pressure responsive microphone



Aug. 28, 1951 H. F. OLSON ET AL LINE TYPE PRESSURE RESPONSIVE MICROPHONE Filed June 22, 1950 INVENTORS HARRY F. UL'snN .J an sr cm ATTORNEY Patented Aug. 28, 1951 LINE TYPE PRESSURE RESPONSIVE MICROPHONE Harry F. Olson, Princeton, and John Preston,

Metedeconk, N. J assignors to Radio Corporation of America, a corporation of Delaware ApplicationJuneZZ, 1950, Serial No. 169,574

This invention relates to sound translating apparatus, and more particularly to an improved line type, pressure responsive microphone.

In'the broadcast field where sound apparatus is often close to the sound source, there is a need for an instrument of -a size such that itis relatively inconspicuous and yet has excellent sound reproducing qualities. For example, in television broadcasting where the microphone is located close to a performer, it should not obstruct the view of the performer from the camera. For some of theseapplications, the pressure typemicrophone is considered most suitable because the shape of the response frequency characteristic of this type of microphone does not vary with the distance from the sound source. It is a primary object of the present invention to provide an improved microphone which will not materially interfere with the view of a performer when it is closely disposed in front of him.

Another object of the present invention is to provide an improved, small, pressure type microphone having excellent sound reproducing qualities and which can be disposed remotely from the sound pick up point. 1

It is also an object of the presentinvention to provide a small microphone having a high order of sensitivity over a wide range ofuseful sound frequencies which isparticularly usefulin motion picture, television, or other stage productions where it is desired to keep the microphone as inconspicuous as possible.

Another object of the present invention isto provide a pressure responsive, electroacoustic translating device which is highly efiicient and so compact in structure that it is easily portable and relatively inconspicuous.

In accordance with the present invention, the microphone comprises a magnetic field structure defining an air gap within which a flexible, ribbon or strip conductor is mounted for vibration in well known manner. Long tubes or conduits are arranged to form chambers on opposite sides of the air gap thereby to isolate the strip from the sound pick up point. The tube on one side of the strip comprises a sound inlet pipe for transmitting sound waves to the conductive strip. The cross sectional area throughout the length of the sound transmitting tube is made substantially equal to the cross sectional area of the air gap so that the surge acoustical resistance of the sound transmitting tube is substantially equal to the terminating acoustical resistance of the air gap. The other 8 Claims. (Cl. 179-115.5)

tube comprises a pipe of relatively infinite length for dissipating substantially completely the sound waves transmitted into it. In order to accentuate the high frequency response, a small horn may be provided for mounting on the open end of the sound inlet pipe.

The novel features characteristic of the present inventiongas well as additional objects and advantages thereof, will be better understood from the following detailed description when read in connection with the accompanying drawing, in which:

.Figure 1 is a front elevation of one form of microphone in accordance with the present invention,

Figure 2 is an enlarged, central longitudinal section of the microphone shown in Figure 1,

Figure 3 is an enlarged, side view of the con nector shown inFigure 2 between the diaphragm chamber and the acoustic labyrinth.

Figure 4 is a front View of the connector illustrated in Figure 3,

Figure 5 is an end view of the connector shown in Figures 3 and 4, looking in the direction of the arrows 5- 5 in Figure 4, and.

Figure 6 is a view similar to Figure 5 looking in the direction of the arrows 65 in Figure 4 and showing the position of the conductive strip with respect to the rectangular opening of the connector.

Referring more particularly to the drawing, wherein similar reference characters designate corresponding parts throughout, there is shown a microphone l for converting sound waves into corresponding electrical variations in accordance with a preferred embodiment of the present invention. The microphone I comprises a light weight housing 3 in which are mounted a sound translating unit 5, a sound transmitting tube 1, an acoustic labyrinth 9, and an output transformer unit II.

The sound translating unit 3 is of the pressure responsive type and includes a flexible ribbon diaphragm or strip I3 mounted in a mag-= netic field for vibration in response to. sound waves impinging thereon. The ribbon diaphragm I3 comprises a corrugated strip of electroconductive material, such as aluminum foil. I'he magnetic field is provided by a structure of generally cylindrical shape and comprises a pair of semicylindrical permanent magnets IS, an annular base plate ll, and a pair of semicylindrical pole pieces or plates I 9. Each of the magnets I5 has one end thereof fastened to the base plate I1, and they are arranged so that they will be disposed in spaced apart relation with their flat sides facing each other and sub= stantially parallel. The pole pieces I9 are mounted on the opposite ends of the magnets with their plane edges or faces disposed in closely spaced, parallel relation to provide an elongated air gap of rectangular cross section therebetween. The ribbon diaphragm It is fastened at the ends thereof by suitable clamping means (not shown) of a kind which Will support the ribbon centrally within the air gap with small clearance between the side edges of the ribbon and the adjacent pole piece plane edges so that it divides the air gap chamber into separate compartments. Thus, the ribbon diaphragm is supported within a magnetic field in the air gap for vibration in well known manner.

The sound transmitting tube 1 comprises an elongated pipe or conduit having an unobstructed bore of relatively small circular cross section. One end of the pipeis open to the ambient for picking up sound waves for transmission through the pipe to the ribbon diaphragm I3. A portion of the opposite end of the pipe defines a throat Ia the cross section of which varies from a circular shape having a diameter corresponding to that of the pipe I to a rectangular shape at the end of the pipe adjacent the air gap. The shape of the pipe throat portion la is such that it converges from its upper end to its lower end in one plane, and diverges from its upper end to its 'lower end in a plane normal to the first mentioned plane considered in a direction along the longitudinal axis of the transmitting tube or pipe I. The end of the pipe throat portion defines a rectangular orifice which opens into and surrounds the air gap so that it forms a closed chamber. Thus, the pipe I closes off the air gap on one side of the conductive ribbon I3 directly from the ambient. 1

In order to hold the throat end of the pipe in alignment with the air gap and to assist in supporting the pipe I so that the longitudinal axis thereof passes through the air gap at right angles to the longitudinal and transverse axes of the ribbon I3, an annular plate ZI is fastened to the throat end. Insulating material 23 is disposed between the plate 2! and the pole pieces I9 and the plate is secured to the pole pieces I9 by any suitable fastening means.

Coupled to the rear of the air gap or diaphragm chamber on the side opposite the sound transmitting pipe I is an acoustic labyrinth 9. The acoustic labyrinth comprises a pipe of relatively great length so that it functions as an acoustical resistance terminating the back of the ribbon I3 to make it responsive to the pressure component of a sound wave in well known manner. The pipe or conduit 9 is made in the form of a single cylindrical block or unit having a plurality of longitudinally disposed, communicating tubes or bores 25. The bores 25 are interconnected to form a continuous passage so that the unit comprises an acoustic labyrinth of the type more particularly shown and described in Olson Patent No. 2,271,988.

In order to close off the air gap at the rear of the ribbon I3 and connect it to the acoustic labyrinth 9,. a labyrinth connector 21 is provided. The connector 21 is constructed similarly to the throat portion of the pipe I so that a rectangular orifice 29 is defined at one end and a circular orifice 3I' at the opposite end. The rectangular end of the connector 21 abuts the pole pieces I9 and is cemented or otherwise fastened thereto to hold the base plate I1.

The labyrinth connector 2! and the acoustic labyrinth bores 25 are filled with suitable acoustic damping material 35 such as loosely distributed hair felt, or the like, thereby to simulate the effect of a loading pipe of infinite length, as

described in the above mentioned Olson patent.

By making the surge acoustical resistance of the line or sound transmitting pipe I coupling free space to the ribbon I3 substantially equal to the terminating acoustical resistance at the air gap, there will be substantially no deterioration in the response frequency characteristic of the microphone l resulting from the line coupling. This has been accomplished by making the cross sectional area of the bore of the sound transmitting tube 1, throughout the length thereof; substantially equal to the area of the air gap in a plane corresponding with that including the longitudinal and transverse axes of the ribbon l3.

The output transformer unit I I is mounted below the acoustic labyrinth 9 and comprises an output transformer 31 mounted in any convenient manner within a casing 39. An opening 4| in the upper wall of the casing 39 is in registry with another opening 42 in the bottom of the acoustic labyrinth 9 for communication with the continuous pipe. Thus, the effective length of the acoustic labyrinth is extended by the space within the transformer casing 39. The transformer casing is also filled with acoustic damping material of the kind disposed in the bores 25.

The various parts or units are arranged to slidably fit in the microphone cylindrical housing 3 in close fitting relation with the interior walls thereof. The parts are disposed one above the other starting with the transformer unit It at the lower end of the housing. The acoustic labyrinth unit 9, the sound translating unit 5 and the sound transmitting pipe I are mounted above the transformer casing 39 in the .order named. The housing 3 is closed at the bottom end thereof and provided with a suitable coupling 43 for connecting the microphone I to an external elec trical circuit. The opposite end of the microphone housing 3 is tapered to conform substantially to a diameter slightly larger than that of the sound transmitting pipe! so that it forms a long, slender tube.

In case it is desirable to accentuate the high frequency response, this can be done by coupling a small horn 45 to the open end of the sound transmitting tube 1. The small horn 45 is tapered outwardly toward its free end at a rate of flare which will provide the high frequency response characteristic desired. For the purpose of preventing foreign matter from entering the sound transmitting tube, a screen 4? is mounted on the disposed close to a sound source it will be relatively inconspicuous. I

Although only a single, preferred embodiment.

of the present invention has been illustrated and described herein, it shouldbe obvious to those persons skilled in the art thatmodifications and changes are possible within the spirit of the invention. For example, the sound transmitting tube need not be straight butmayhave the sound pickup end angularly disposed with respectto the body of the instrument. In such a case, the instrument may be mounted on its side on a support for placing it on a desk or table with the tube pickup end disposed upwardly. Other changes of like nature will undoubtedly suggest themselves to those skilled in the art. Therefore, it is desired that the particular form of the present invention described herein shall be considered as illustrative and not as limiting.

What is claimed is:

1. An electroacoustical translating device comprising a magnetic field structure including means providing a closed chamber, sound transmitting conduits disposed on opposite sides of and opening into said chamber, and a flexible, electro-conductive ribbon diaphragm extending across said chamber substantially cutting oif communication between said conduits, said con duits being substantially equal in cross sectional area at points therein in proximity to said chamber, said cross sectional area being substantially equal to the area of said chamber in the plane of said ribbon diaphragm, one of said conduits having an unobstructed bore terminating in an open end remote from said chamber for picking up sound waves for transmission through said one conduit to said ribbon diaphragm, the cross sectional area of said bore throughout the length thereof being substantially equal to said chamber area.

2. The invention set forth in claim 1 and wherein a relatively small horn is disposed on said one conduit open end for accentuati ng response to high frequency sound waves.

3. An electroacoustic translating device comprising a magnetic field structure includin spaced magnetic pole pieces defining an elongated air gap, an elongated conduit having an unobstructed bore throughout a greater portion of the length thereof for directing sound waves to said air gap, one end of said conduit abutting said pole pieces and defining a throat portion which converges in one lane toward the end thereof adjacent said air gap and diverges in a plane normal to said one plane toward said end adjacent said air gap considered in a direction along the longitudinal axis of said conduit, the

end of said throat portion abutting said pole pieces defining an elongated slot opening into said air gap, an elongated strip of flexible conducting material in and extending lengthwise of said slot with relatively small, clearance between each side edge thereof and the pole piece adjacent thereto, the cross sectional area of said bore throughout the length of said conduit being substantially equal to the area of said air gap in the plane including the longitudinal and transverse axes of said elongated strip, and means including said elongated conduit for closing off said air gap directly from the ambient.

4. An electroacoustic translating device comprising a magnet having spaced apart pole pieces providing an air gap therebetween, a conduit having its longitudinal axis passing through said air gap for directing sound waves to said air gap, one end of said conduit abutting said pole pieces to close oif said air gap directly from the ambient from one side thereof, said conduit opening directly into said air gap through an elongated.

rectangular orifice of substantially the same cross sectional area as said air gap, an elongated strip of flexible conducting material in and extending lengthwise of said air gap with its side edges in closely spaced relation to said pole pieces, said conductive strip having its longitudinal and transverse axes extending normal to said conduit longitudinal axis, the cross sectional area of said conduit throughout the length thereof being substantially qual to the cross sectional area of said air gap in the plane including the longitudinal and transverse axes of said conductive strip, and means closing off the remainder of said air gap from the ambient.

5. An electroacoustic translating device comprising spaced magnetic pole pieces defining an elongated air gap, a flexible elongated strip of conducting material mounted between said pole pieces centrally within said air gap for vibratory movement in response to sound waves impinging thereon, said conductive strip being disposed with relatively small clearance between the side edges thereof and the pole pieces adjacent thereto, a conduit the longitudinal axis of which passes through said air gap at right angles to the longitudinal and transverse axes of said strip, an end portion of said conduit comprising a throat having an open end abuttin said pole pieces to close off said air gap on one sid of said conductive strip directly from the ambient, said conduit having an unobstructed bore throughout the length thereof communicating with said throat, th cross sectional area of said bore being substantially equal to the cross sectional area of said air gap in a plane normal to said conduit longitudinal axis, and means closing off said air gap on the opposite side of said conductive strip directly from the ambient.

6. The invention set forth in claim 5 wherein said means closing off the opposite side of said air gap provides an acoustical resistance terminating the back of said conductive strip and comprises a pipe of relatively great length, the cross sectional area of said pipe being substantially equal to that of said unobstructed bore.

7. An electroacoustic translating device comprising a closed diaphragm chamber, a diaphragm comprising a thin, corrugated strip of conductin material mounted in said chamber for vibration in response t sound waves impinging thereon, a sound transmitting pipe having an unobstructed bore of relatively great length open at one end and in direct open communication at its opposite end with said chamber, the cross sectional area of said bore throughout the length thereof being substantially equal to the cross sectional area of said diaphragm chamber in the plane of said strip, a second pipe of relatively great length in direct open communication at on end thereof with said diaphragm chamber, the cross sectional area of said second pip at said open end being substantially equal to the cross sectional area of said first mentioned pipe, sound absorbing material in said second pipe for dissipating substantially completely sound waves which pass into said pipe, and means for converting vibrations of said diaphragm into electrical oscillations.

8. An electroacoustic translating device comprising magnetic means providing a magnetic field within a closed chamber, a flexible, electroconductive member mounted in said chamber for movement in response to sound waves impinging thereon, said flexible member being disposed centrally in said chamber and dividing said chamber into separate compartments, a tubular member opening directly into one of said compartments for directing sound waves to one side of said flexible member, said tubular member 5 being so dimensioned that the surge acoustical resistance thereof is substantially equal to the terminating acoustical resistance of said chamber, and means communicating with the other of said compartments providing an acoustical REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,032,389 An'derson Mar. 3, 1936 2,228,886 Olson Jan. 14, 1941 2,271,988 Olson Feb. 3, 1942 

